Identification of serine 245 as a novel determinant of the transcriptional regulatory activity of the CREB coactivator CRTC1

Activity-dependent changes in gene expression are implicated in the pathogenesis of Huntington's disease (Gambazzi et al., 2010; Spektor et al., 2002; Luthi-Carter et al., 2000). One of the most important facets of transcriptional dysfunction in HD appears to be the decreased activity-dependent regulation of brain derived neurotrophic factor (BDNF) promoter IV. The CREB coactivator CRTC1 has recently been established to be an important regulator of activity-dependent neuronal gene expression, including the regulation of BDNF (Espana et al., 2010; Zhou et al., 2006). We therefore set out to explore the hypothesis that CRTC1 regulation might be deficient in HD-affected cells. This goal led us to better understand the mechanisms of CRTC1 regulation. We assessed the role of post-translational regulation on the activity of CRTC1 by analogy to another CRTC protein, CRTC2. In the present work, we explored the role of two candidate phosphorylation sites, Ser 151 and Ser 245, in regulating the transcriptional activation mediated by CRTC1 in HEK293T cells and in primary cortical neurons. The involvement of calcineurin in the regulation of CRTC1 activity was established by monitoring CRTC1 intracellular localisation after treatment with cyclosporine A. By mutating serines 151 and 245 to alanines we mimicked the constitutively dephosphorylated state at the candidate positions. We then assessed whether these CRTC1 mutations would show a constitutively active-like behaviour. Expression of CRTC1 constructs in HEK293T cells by transient transfection or in primary neurons via lentiviral gene delivery allowed us to explore whether these serines would affect CRTC1’s intracellular localisation, its activation of gene expression from CRTC1-CREB dependent promoters and its ability to protect neurons from mutant huntigtin-related toxicity. We observed increased nuclear localisation of CRTC1 mutants (S151A, S245A, and S151A+S245A), with the mutation of both serines showing the greatest effect. Whereas S151A showed a significantly induced transcription of CRTC1-CREB target genes, including BDNF, S245A did not produce this effect. Of great interest, however, S245A mutation in combination with S151A produced a significantly greater induction of gene expression than S151A alone. Moreover, the S151A+S245A mutation showed significant neuroprotection in a striatal neuron model of HD. Our observations support the involvement of both Ser151 and Ser245 in the regulation of CRTC1 activity. Taken together with the neuroprotection results, these data also support the novel hypothesis that CRTC1 is a mediator of the HD-related deregulation of neuronal gene expression.